The prevalence of obesity exceeds 25% among adults, with the prevalence among men being only slightly lower than among women. Recent studies have demonstrated that exercise plays a role in both weight loss and long term maintenance of that reduced weight. During weight loss, exercise increases the rate of fat loss. The increase, however, has been hard to explain because it exceeds that predicted from the energy costs of the exercise. Our current studies show that the addition of exercise to obesity treatment in women has multiple effects and the sum of these explains the enhanced fat loss. Exercise acts to both improve dietary compliance and minimizes energy restriction related decreases in energy expended in daily living. Our current studies have also provided preliminary evidence of a threshold level of total daily energy expenditure for maintenance of weight among previously obese women. The threshold corresponds to 1.75 times resting metabolic rate. For a 63 kg woman, this requires the addition of 70 min brisk walking or 35 min of aerobic exercise each day to a sedentary life style. Although the number of studies is quite small, exercise has also been identified as a key component for the treatment of obesity among men. The mechanism through which exercise acts in men, however, is unknown. We hypothesize that exercise acts through improved dietary compliance in men and that exercise is associated with positive mood and self-efficacy. We also hypothesize that there will be a critical level of physical activity to maintain weight in men. To test these hypotheses, we will measure total daily energy expenditure, dietary energy intake, and fat loss in men [performing aerobic exercise and resistance training during a balanced hypocaloric diet. These will be compared with diet alone, exercise alone and diet plus stretching to control for the increased contact of the supervised exercise programs. Both weight loss and one year weight maintenance will be investigated. We hypothesize that these effects will be mediated through increased whole body lipidysis and fatty acid oxidation for up to several days after the exercise bout which has not previously been investigated in humans. We will test this using the constant labeled glycerol infusion model in trained and untrained individuals. Because men select exercise for weight loss and because there are few studies from which recommendations can be drawn, results from this study will have important ramifications for obesity treatment.